Pressure-contactable power semiconductor module

ABSTRACT

A power semiconductor module capable of pressure contact, with a base plate and a cover plate, is provided. The power semiconductor module comprises at least one semiconductor device with a first main terminal and with a second main terminal, which is in electrically conducting connection with the base plate, and also at least one spring element, which is arranged between the first main terminal and the cover plate. An electrically conducting connection between the first main terminal and the cover plate is led through an inner region of the spring element.

TECHNICAL FIELD

The invention relates to the field of power electronics. It concerns apower semiconductor module capable of pressure contact according to theprecharacterizing clause of the first claim.

PRIOR ART

Such a power semiconductor module is already described in the laid-openpatent application DE 199 03 245 A1. The power semiconductor moduleconcerned is of the so-called pressure contact type with at least onesemiconductor device, as shown in FIG. 1. A first main terminal 31 ofthe semiconductor device 3 is connected by at least one contact element8 in an electrically conducting manner to a cover plate 2. With a secondmain terminal 32, the semiconductor device 3 is arranged on a base plate1.

For reliable contacting in such power semiconductor modules, pressuresof the order of magnitude of 1 kN/cm² are required. To generate thesepressures while at the same time compensating for variations in thethickness of the semiconductor devices, in particular in the case ofmodules with a number of semiconductor devices, the contact element 8has at least one spring element 4, which is generally formed as a spiralspring or as a stack of cup springs.

One difficulty is that of combining the spring element with a suitablecurrent lead in the contact element. In DE 199 03 245 A1 this is solvedby the current lead being led around the spring element by means of aflexible connecting clip or wire 81. For this reason, the connectingclip 81 must have a minimum length, which is determined by thedimensions of the spring element. In addition, the maximum cross sectionof such a connecting clip 81 is limited, since otherwise its mobilitywould be restricted. The two requirements just mentioned result in alower limit for the thermal and electrical resistance of the connectingclip, which must be maintained, with respect to given dimensions of thespring element. For this reason, it must be ensured by selecting ordesigning the spring element that dimensions that permit tolerableresistance values of the connecting clip 81 are achieved.

In order to keep the semiconductor module itself, or else a possiblyhigher-level system, fully operational in the event of failure of one ormore semiconductor devices 3, it is necessary in many cases for adefective semiconductor device 3 to switch over into a stableshort-circuit mode (“short-circuit failure mode”, SCFM), in which apermanent, electrically conducting contact with lowest possibleresistance and greatest possible current capacity exists between thefirst and second main terminals. If the SCFM occurs in the case of asemiconductor device in a pressure-contact power semiconductor module, acurrent through the module flows completely through the correspondingsemiconductor device 3 and the connecting clip 81 of the associatedcontact element 8. Customary currents in this case lie in the range of afew kiloamperes. On account of the aforementioned thermal resistance ofthe connecting clip 81, temperatures far in excess of 200° C. canconsequently occur in the semiconductor device 3. On the device side,the connecting clip 81 is heated up to similarly high temperatures as aresult, which leads to severe stressing. The contact element in DE 19903 245 A1 additionally has an internal pressure contact between theconnecting clip 81 and a pressure stamp 9. As a result, the total numberof pressure contacts in the power semiconductor module is increased,whereby overall electrical and thermal resistances are increased.

If a power semiconductor module comprises more than one contact element8, it must be ensured when loading it with components that theconnecting clips 81 do not touch one another. For this reason,correspondingly large distances must be provided between the contactelements 8 during production and/or correct mutual alignment of thecontact elements 8 must be ensured during loading with components.

SUMMARY OF THE INVENTION

The present invention is based on the object of providing a powersemiconductor module of the type stated at the beginning which manageswithout a connecting clip led around a spring element of a contactelement for the purpose of current leading. The invention is also basedon the object of eliminating existing restrictions in the selection anddesign of a spring element with regard to its dimensions. The inventionis further based on the object of minimizing the number of necessarypressure contacts in the power semiconductor module. Finally, theinvention is based on the object of permitting a more compactconstruction of the power semiconductor modules and simplifying loadingwith components.

This object is achieved by a power semiconductor module according toclaim 1. According to the invention, a combination comprising a currentlead and a spring element is modified in such a way that an electricallyconducting connection between a first main terminal of a semiconductordevice and a cover plate is led through an inner region of the springelement. Current and heat are consequently no longer led around thespring element on the outside but are led within the spring element. Inthis way, both a minimum cross section and an average cross section ofthe conducting connection are increased in comparison with the priorart. Regions of the connection with cross-sectional values close to theminimum cross section are shortened. Altogether, a clear reduction inthe electrical resistance, and in particular the thermal resistance, isachieved in this way. The inner region is understood hereafter asmeaning a recess, opening, lead-through or the like that passes throughthe spring element substantially parallel to an intended direction ofcompression of the spring element.

Furthermore, the invention allows larger spring elements to be used. Inaddition, internal pressure contacts between component parts of thecontact element can be eliminated. The elimination of the connectingclip allows a compact construction and simple loading with components ofthe power semiconductor modules according to the invention.

These and further objects, advantages and features of the invention areobvious from the more detailed description which follows of a preferredexemplary embodiment of the invention in conjunction with the drawings.

BRIEF DESCRIPTION OF THE FIGURES

In the schematic drawings:

FIG. 1 shows a power semiconductor module according to the prior art,

FIG. 2 a shows a section through part of a power semiconductor moduleaccording to the invention in a first embodiment, without the effect ofa pressure contact force,

FIG. 2 b shows the power semiconductor module from FIG. 2 a, under theeffect of a pressure contact force,

FIG. 3 shows a section through part of a preferred development of apower semiconductor module according to the invention and

FIG. 4 shows a section through part of a power semiconductor moduleaccording to the invention in a further preferred configuration.

The reference numerals used in the drawing and their meaning arecompiled in the list of designations. In principle, the same parts areprovided with the same reference numerals.

WAYS OF IMPLEMENTING THE INVENTION

FIG. 2 a schematically shows a section through part of a powersemiconductor module according to the invention in a first embodiment,without the effect of a pressure contact -force. A semiconductor device3 lies with a second main terminal 32 on a base plate 1. A contact stamp5 lies with a contact area on a first main terminal 31 of thesemiconductor device 3. A stamp neck 52 of the contact stamp 5 protrudesin this case into an inner region 44 of a spring element 4. In the caseof this exemplary embodiment, the spring element 4 is a spiral spring.The inner region 44 is in this case a substantially cylindrical region,which is enclosed by turns of the spiral spring. On an end face 51 ofthe contact stamp 5 that is facing the cover plate 2, a deformableconnecting element 6 is attached by means of a fixed, integralconnection 56 and contacts a cover plate 2. The contact stamp 5 and theconnecting element 6 form a current lead, the flexible part of which islaid locally over the spring assembly. By increasing the cross sectionof the contact stamp 5 at the end of the stamp neck 52 that is facingthe base plate and a suitable form of the connecting element 6, thespring element 4 can transfer the force from the contact stamp 5 to theconnecting element 6 and vice versa. In comparison with a connectingclip 81 used in the prior art, the connecting element 6 can beshortened, while at the same time increasing its cross section, whichleads to a clear reduction both in the electrical resistance and in thethermal resistance. This also results in a better distribution of thethermal resistance between the contact stamp 5 and the connectingelement 6. Consequently, a homogeneous temperature gradient is obtainedwhen the power semiconductor module is in operation, a first temperatureof the connecting element 6 remaining clearly below a second temperatureof the semiconductor device 3. The fixed, integral connection 56 betweenthe end face 51 and the connecting element 6 dispenses with the need foradditional internal pressure contacts, which are present in the contactelement according to the prior art. In the embodiment shown, the numberof pressure contacts that are located between the cover plate 2 and thesemiconductor device 3 can be reduced to two. The fixed, integralconnection 56 is preferably a welded connection, but the connectingelement 6 may also be attached to the end face 51 in some other manner.Likewise advantageous for example is a soldered connection or alow-temperature connection. It is also of advantage, however, if duringproduction the connecting element 6 and the contact stamp 5 are alreadyproduced directly from one piece. Screwed or riveted connections canalso be advantageously used. The contact stamp 5 and the connectingelement 6 are preferably formed such that they are rotationallysymmetrical with respect to an axis of rotation A, but other forms canalso be used with advantage.

In the case where a number of power semiconductor modules are assembledto form a stack or a power semiconductor module is installed in ahigher-level system, electrical contacting takes place via the baseplate 1 and the cover plate 2, which for this purpose are subjected topressure, whereby the power semiconductor module is compressed by adistance Ax, as shown in FIG. 2 b. The force on the base plate 1 orcover plate 2 is transferred by the spring element 4 to the connectingelement 6 or the contact stamp 5. Preferably, there are lateral modulewalls (not shown in the figure) that determine a maximum compressibilityAxon of the power semiconductor module. If a linear spring with a springconstant k is used, the force F on the connecting element 6 and thecontact stamp 5 in a completely compressed state of the powersemiconductor module is given by F=kΔx_(max). Nonlinear springs can alsobe advantageously used.

FIG. 3 schematically shows a section through part of a preferreddevelopment of a power semiconductor module according to the invention.In the case of this embodiment, a pressure-exerting element 7 is presentbetween the spring element 4 and the connecting element 6. Thepressure-exerting element 7 has in this case a cup form and preferablyconsists of insulating material. The stamp neck 52 of the contact stamp5 is led through an opening 71 in a bottom wall of the pressure-exertingelement 7. A pressure-exerting element 7 of a suitable form permits auniform distribution of the force transferred to the connecting element6, independently of the type and form of the spring element 4.Furthermore, the pressure-exerting element 7 can be advantageously usedfor increasing the usable spring excursion.

In a preferred configuration of the invention, the spring element 4 isformed by a stack of cup springs, the individual cup springsrespectively having at least one bore or hole and being assembled toform a spring element in such a way that the bores or holes produce aninner region suitable for leading through an electrically conductingconnection. In the case of such a spring element, properties such as,for example, a length in the relaxed state or else a springcharacteristic can be influenced by the type and/or number of cupsprings used in the stack. This permits increased flexibility in theproduction of the power semiconductor modules according to theinvention, because a wide range of power semiconductor modules withdifferent pressure contact behavior can be produced with a small numberof different cup springs. The use of a stack of a number of cup springsalso has the effect of reducing deviations from a desired behavior ofthe spring element caused by production tolerances. In comparison withthe prior art, elimination of the outer connecting clip 81 provides morespace for larger cup springs. This has the consequence that firstly thediameter of the inner region can be greatly increased, which allows anincreased current-leading cross section within the springs, and secondlythe larger cup springs can be bent to a greater extent, with internalstresses remaining small. Consequently, fewer springs are required toachieve a desired spring characteristic.

In a further preferred configuration of the invention, the springelement 4 is assembled from individual springs 42 that are arranged inparallel and the first ends of which are mounted on a first fasteningring 41 and the second ends of which are mounted on a second fastening43, as can be seen in FIG. 4. A region between the individual springs 42forms the inner region 44 of the spring element. Here, too, propertiesof the spring element 4 can be influenced by changing the type and/ornumber of the individual springs 42 used in the spring element 4.Instead of the first fastening ring 41, there may advantageously bemeans for fastening the individual springs 42 directly on thepressure-exerting element 7. Similarly, instead of the second fasteningring 43, there may be means for fastening the individual springs 42 onthe contact stamp 5.

In a preferred configuration of the invention, the semiconductor device3 is an individual semiconductor chip, a first and a second mainelectrode respectively forming the first and second main terminals.

In a further preferred configuration of the invention, the semiconductordevice 3 is a submodule which comprises a number of semiconductor chipsconnected in parallel and/or in series and in which the individualsemiconductor chips are interconnected in a suitable way with oneanother and with the first and second main terminals.

In a further preferred configuration of the invention, the powersemiconductor module has between the first main electrode 31 and thecontact stamp 5 and/or between the second main electrode 32 and the baseplate 1 intermediate layers in the form of a foil, plate and/or solderlayer. Presented as an example of such an intermediate layer are platesthat are adapted in their thermal expansion to the semiconductor device3 and are produced for example from Mo, Cu, or Mo—Cu, Al—C, Cu—C orAl—Si—C composites.

List of Designations

-   1 Base plate-   2 Cover plate-   3 Semiconductor device-   31 First main terminal-   32 Second main terminal-   4 Spring element-   41 First fastening ring-   42 Individual springs-   43 Second fastening ring-   44 Inner region-   5 Contact stamp-   51 End face of the contact stamp-   52 Stamp neck-   56 Fixed, integral connection-   6 Connecting element-   7 Pressure-exerting element-   71 Opening-   8 Contact element-   81 Connecting clip or wire-   9 Pressure stamp

1. A power semiconductor module capable of pressure contact, comprisinga base plate, a cover plate, at least one semiconductor device with afirst main terminal and with a second main terminal, which is inelectrically conducting connection with the base plate, at least onespring element, which is arranged between the first main terminal andthe cover plate, wherein an electrically conducting connectioncomprising a contact stamp between the first main terminal and the coverplate is led through an inner region of the spring element, the springelement serving for the force transfer to the cover plate and to thecontact stamp.
 2. The power semiconductor module as claimed in claimwherein the contact stamp has on an end face facing the cover plate adeformable connecting element, which is in electrically conductingconnection with the cover plate.
 3. The power semiconductor module asclaimed in claim wherein the connecting element is attached on the endface of the contact stamp by means of a fixed, integral connection. 4.The power semiconductor module as claimed-in claim wherein a force canbe transferred between the contact stamp and the connecting element bythe spring element.
 5. The power semiconductor module as claimed inclaim wherein for transferring the force between the contact stamp andthe connecting element, a pressure-exerting element is provided betweenthe contact stamp and the connecting element.
 6. The power semiconductormodule as claimed in claim wherein the pressure-exerting element isformed substantially as a cup and a stamp neck of the contact stamp isled through an opening in the bottom of the pressure-exerting element.7. The power semiconductor module as claimed in claim 1, wherein thepressure-exerting element consists of electrically insulating material.8. The power semiconductor module as claimed in one claim 1, wherein thespring element is formed by a stack of cup springs.
 9. The powersemiconductor module as claimed in claim 1, wherein the spring elementhas a number of individual springs arranged in parallel.
 10. The powersemiconductor module as claimed in claim 1, wherein between the firstmain electrode and the contact stamp there is at least one electricallyconducting layer.
 11. The power semiconductor module as claimed in claim1, wherein between the second main electrode and the base plate there isat least one electrically conducting layer.